Plant assays and avoidance tests with collembola and earthworms demonstrate rehabilitation success in bauxite residue

Bauxite residues are a by-product of alumina manufacture from bauxite ore and are commonly disposed of in purpose-built bauxite residue disposal areas (BRDAs). Revegetation is viewed as the most effective way to landscape and rehabilitate closed BRDAS and physicochemical assessment remains the primary indicator of rehabilitation success. Little is known about the ability of keystone mesofaunal species to colonise and establish in these environments yet the long-term success of rehabilitation is dependent on residues becoming suitable habitats for such groups. Using six different residue treatments (untreated, leached, organic application, organic amended, and two revegetated field treatments) together with OECD test soil, this study assessed the characteristics of residues with plant germination and seedling development using the Rhizotest™ approach with Lepidium sativum, Sinapis alba, and Sorghum bicolor. Avoidance tests with soil mesofauna Eisenia foetida together with growth and reproduction tests for Folsomia candida were conducted to determine possibility of inhibition in residue soils. Unamended residue is inhibitory to plant growth and mesofaunal establishment. Amendment improves the physicochemical properties of the residue, and data revealed that both gypsum and organic addition is required to promote conditions favourable to plant growth and mesofauna establishment. Earthworms avoid residues with elevated Na content but will choose substrate with high soluble Ca content. F. candida preferentially moves to residues from field treatments, and both mortality and reproduction rates are comparable or superior to OECD soil. On the basis of these assays, we propose that bauxite residue can be transformed to a soil-like medium capable of supporting keystone species.